Passive restraint systems such as air bags or automatic seat belt tensioners have experienced increased usage in vehicles to protect passengers during frontal collisions. These types of occupant protection and safety devices require no extra action by the passenger to achieve protection. Rather, the passive restraint automatically activates in the event of an activation worthy impact event. To determine if a given event is activation worthy, an impact sensing system in the vehicle detects impacts and discriminates between activation worthy and nonactivation worthy impacts.
One type of impact sensing system uses a plurality of threshold switches in the front region of the vehicle. These switches send a signal for inflating an air bag if a high impact event is severe enough to close the switches. Mechanical sensor-based systems of this type usually rely on sensor redundancy to minimize the negative effects of any sensor malfunction which may occur. This requires a large number of switches within the vehicle, each of which must be calibrated separately, increasing the overall complexity of the impact sensing system and its calibration process. To ensure proper functioning, the threshold switches must be located in strategic places in the vehicle where they have the best opportunity to detect and discriminate between various types of impacts. Determining these locations during calibration of the impact sensing system requires extensive impact testing and studying of impact effects on the vehicle to determine the best placement for the threshold switches.
Another type of impact sensing system uses a single point impact sensor instead of multiple switches. This type of sensing system has an accelerometer located in the passenger compartment of the vehicle that constantly monitors the vehicle's acceleration and senses any sudden deceleration. The output of the accelerometer is continuously analyzed to determine if and when deceleration occurs and if the deceleration is caused by an impact that is severe enough to require activation of the air bag or other passenger restraint. This type of impact sensing system is easier to calibrate because the sensor does not involve a large number of parts that need to be calibrated individually. Only the accelerometer and the discrimination circuitry need to be calibrated.
Most sensors require application of a measurand on the sensor during calibration. A measurand is defined as the type of value that the sensor is designed to measure. For the impact sensor, the measurand is an acceleration value, but a measurand can be any type of physical or chemical phenomenon. Calibration is one of the most expensive and time-consuming steps in sensor manufacture because application of the measurand requires a high degree of precision.
In many impact sensing systems, lasers trim thin film resistance networks in the sensor during manufacture to achieve proper calibration. However, this type of calibration must take place as an intermediate step in the manufacturing process, not the final step, since the sensor package must still be open during calibration to give the laser beam access to the resistor network. The measurand must then be applied to this incomplete device to calibrate it. After laser trimming, the lid is attached to completely enclose the sensor, and the measurand is applied a second time to confirm the accuracy of the sensor calibration.
This method requires additional manufacturing steps to take place after the initial calibration, such as solder seal, epoxy core, or burn-in. Consequently, the sensor undergoes additional stresses after calibration through these subsequent manufacturing processes. The stresses undermine the integrity of the first calibration and increase the likelihood of damaging the sensor prior to a final sensor confirmation test.
Several sensor calibration methods have been developed to minimize the problems associated with conventional calibration processes. U.S. Pat. No. 4,669,052 to Bianco describes a method and apparatus for calibrating a sensor output that eliminates conventional trimming and other internal calibration techniques that change the physical characteristics of the sensors.
This calibration method is accomplished by providing a database, which is empirically prepared for each particular sensor, to relate the sensor output to known environmental influences. The database is then stored in the sensor memory. Slope values, which indicate the slope between various test points, can also be calculated and stored in the database to define the relationships between external event values and sensor output values. With this type of calibration, however, the empirically obtained data is assumed to be correct and is used as the reference point for subsequent measurements. There is no verification provision in this calibration method to conduct a final test after calibration to compare the sensor output with a fixed standard to ensure that the calibrated output of the sensor is indeed the desired output.